Lasers are widely-used in various industrial material processing techniques due to the capability to provide high energy concentration in a spot by focusing of a laser beam. The physical effects happening while interaction of material and concentrated laser energy can lead to necessary processing effects; such as for example, melting and evaporation of metals and non-metallic materials like glass, Carbon Fiber Reinforced Polymers (CFRP) are important in laser welding and cutting applications. However, material disruption can occur due to multiphoton absorption or thermal stress is used in laser technologies of cutting, chamfering, singulation, dicing, drilling of brittle materials, for example various types of glasses, sapphire, silicon, other transparent and semi-transparent materials, as well as multilayer structures.
Focusing of a laser beam on a workpiece surface or inside bulk material is a popular used approach in laser processing technologies. Therefore, the task of optimizing laser radiation focusing in order to improve the performance of laser technologies and increase the productivity is an important industrial task.
A method of processing the brittle materials such as glass or sapphire, through internal focusing and creating material disruption or modification is described in U.S. Pat. Nos. 7,749,867 and 8,183,131 to Fukuyo et al. which include scanning of focused laser beams along a processing path there is created a processed region inside the bulk material, from which the material separation starts while further processing.
Among methods attempting to improve the quality and controllability of the processing there is used a method of multi-step processing described in U.S. Pat. Nos. 6,992,026, 7,396,742, 7,547,613, 7,592,238, 7,615,721, 7,732,730, 7,825,350, 8,227,724, 8,946,591, 8,946,592, 8,969,752, 8,969,761 all to Fukuyo et al.; U.S. Pat. No. 8,134,099 to Nakano et al with, the scanning of focused beam is repeated for the same processing path with changing the focusing depth on each step.
This multi-step approach works well especially for thick materials. U.S. Pat. No. 8,890,027 to Fukuyo et al. describes its application to process multilayer materials when separated in depth processed regions to be created. However, there are disadvantages of this method, such as increasing of processing time, high tolerances for alignment between processing paths on each step, and high roughness of the cutting edge reducing the bending strength.
To overcome these disadvantages there are proposed methods of creating elongated processed region by one step of scanning of focused beam. U.S. Patent Application No. 2013/0126573 to Hosseini et al. describes a method of processing by filamentation happening inside bulk material by focusing of ultra-short pulse laser beam. WO/2014/079478 to Bhuyan et al. describes a method of processing using so called Bessel beams, which are characterised by high intensity in proximity to optical axis along certain length being defined by optical system applied. As such, there is created a needle-like processed region along optical axis. Common disadvantages of those methods include complexity and high costs of realization, tough tolerances to specifications of laser radiation, and difficulty to control the depth of the processed region.
Other proposed methods of material processing imply creating multiple processed areas along optical axis by each laser pulse. U.S. Pat. No. 8,389,891 to Bovatsek et al. and DE Patent No. 198 46 368 to Berger et al. describes optical systems based on diffractive optical elements (DOE) and creating two or several focuses. The disadvantages of DOE-based optical systems can include high initial manufacturing costs, limited efficiency and low resistance to powerful laser radiation, especially to widely-used ultra-short pulse lasers, and sensitivity to input beam size and quality. The DOE can efficiently be used with single-mode (TEM00) lasers only. DOE demonstrate low efficiency with multimode lasers. U.S. Pat. No. 8,143,141 to Sugiura et al, U.S. Pat. No. 8,852,698 to Fukumitsu, and U.S. Pat. No. 8,513,567 to Osajima et al. present methods based on multiphoton absorption effect by optimizing laser specifications and parameters of focusing optics to create a microcavity and elongated molten processed region. The tough tolerances to laser specifications and limited number of materials that can be processed are disadvantages of these methods.
Another approach is based on using optical systems creating two or more focuses. U.S. Pat. No. 5,690,845 Fuse describes mirror or lens-based optics for splitting the beam through difference of properties of different parts of an optical component working aperture. The methods can use different dioptric power because of different curvature of optical surface of a lens or a mirror. The manufacturing of such optics can be complex and expensive. Another disadvantage is the presence of junction between parts of the optical surface, which reduces resistance to powerful laser radiation, especially to widely-used ultra-short pulse lasers.
Other solutions include creating multiple focuses can be based on optical systems comprising lens or lenses made from birefringent materials. WO/1991/014189 and U.S. Pat. No. 5,142,411 both to Fiala describe cemented multifocal intraocular lenses (IOL) from polymer materials used to extend the eyesight depth of field. However, the IOL cannot be applied in laser material processing because these optics are cemented and made from polymers. U.S. Pat. No. 6,057,970 to Kim et al. describes an optical system for semiconductor lithography with a birefringent lens to create two focuses and extend the depth of field. Another example of using a birefringent lens is a two-focus imaging system described in U.S. Pat. No. 4,566,762 to Kato.
U.S. Pat. No. 7,402,773 to Nomaru describes a laser processing machine shown in FIG. 1 having a laser P2, a mirror P4 reflecting a laser beam P3, a birefringent lens P5 being cemented with a glass lens P6, an objective P7 focusing the laser radiation inside a material workpiece P1 in two separate focuses W1 and W2, with the scanning of the focused beam is realized through a scanning device P8. A disadvantage is using cemented optics, which reduces resistance to powerful laser radiation, especially to widely-used ultra-short pulse lasers. The optical system provides two separate focuses only with fixed distance between them. Thus, it is impossible to change the length of elongated processed region by increasing the focuses number and changing distance between focuses. This sets a limitation on thickness of materials to be processed and requires the repetition of processing path in case of thick materials, which reduces the process productivity.
A disadvantage of conventional technical solutions is the absence of ways to compensate aberrations induced by a transparent material workpiece while light focusing inside the said workpiece. Conventional solutions processing effects presume high energy concentration, which is provided by focusing of a laser beam in a small spot, typically few microns size, using optical systems of high numerical aperture (NA). Typically, initial designs of optical systems uses beam focusing in air, while many conventional material processing solutions use the beam focusing inside transparent materials. In case of high NA of focused beam there appears a strong spherical aberration that leads to spreading of laser energy and reducing of efficiency of material processing, which is illustrated in FIGS. 2A, 2B, 2C, 2D, 2E, 2F and 2G.
These figures show results of calculations using optical design software for a laser beam at 1064 nm wavelength being focused by an aberration-free lens with NA0.5 in air and at depth 0.8 mm inside sapphire. Similar results can be achieved for a microscope objective with NA0.5. FIG. 2A shows focusing lines with an aberration-free lens.
FIGS. 2B-2C shows a ray trace near focus, with FIG. 2B showing a focussed beam in air, and FIG. 2B shows a focussed beam inside a sapphire depth (0.8 mm) data.
FIGS. 2D-2E show spot views in a working plane that is a plane of maximum energy concentration with FIG. 2D showing a spot view in air and FIG. 2E showing a spot view inside a sapphire depth (0.8 mm).
FIGS. 2F-2G shows graph diagrams of encircled energy in the working plane.
FIG. 2F shows images that relate to focusing in air. FIG. 2G shows graph diagrams inside sapphire.
The lens can be aberration-free in air. Therefore, while focusing in air, it provides diffraction-limited spot size and maximum achievable energy concentration that can be characterized by a circle diameter where more than 80% of energy is concentrated, and in the considered example the characteristic diameter is 1.5 μm.
When focusing the laser beam using that lens inside sapphire the resulting spot size can be defined by spherical aberration and is several times larger than the diffraction limited one. This has, inevitably, an influence on concentration of laser energy—the characteristic diameter is more than 10 μm and, hence, intensity is about 50 times less than one in case of focusing in air.
Further analysis shows that the higher is optics NA or deeper focusing inside processed materials, the larger is the spherical aberration and, hence, stronger energy dissipation. Evidently this aberration effect should be taken into account in technical solutions implying laser beam focusing inside transparent materials and optical design of focusing optics needs to compensate aberration appearing in case of deep focusing.
From the point of view of modern requirements to material processing using laser beam focusing the conventional techniques are not optimal. As such, there is needed an efficient affordable method, apparatus and system capable to provide variable depth of elongated processed region, reliable control of the processed region depth, variable number of focuses, adaptation to variable focusing depth and compensation of aberration appearing in case of focusing inside bulk material, for the optical system design to comprise air-spaced optical components.